We intend to investigate in depth the structural, functional and metabolic characteristics of ischemic cells and to determine the critical period of acute ischemia during which cell injury can be modified with therapeutic interventions. The changes in serum enzyme levels are an indication of injury to myocardial cells during ischemia and constitute one of the clinical means of detecting myocardial infarction. It is not yet established how enzyme relase is related to ultrastructural defects in plasma and other cell membranes. Enzyme release following early myocardial infaction will be studied in isolated heart preparations. The intracellular membranes play a critical role in the pathogenesis of cell injury, therefore these membranes will be examined with the freeze-fracture technique. The intramembranous particles (70 A) embedded within the membranes and exposed by the freeze-fracture process are reportedly associated, for example, with the calcium transport enzyme in the sarcoplasmic reticulum and will be of significant value in determining the structural competence of the ischemic cells. The identical particles in other membranes may also have specific functions in the cell and their altered location, shape and number may be responsible for the pathogenicity of the membranes.